Figure 6: a) Exploration of the submerged level of water. b) Effect of heating the WS-WEG device. c) Evaluation of different heights of the WS. d) Voltage vs time with changing polarity. e) Effect of relative humidity level on Voc and Jsc. f) Effect of NaCl concentration on Voc and Jsc.
The temperature of the water reservoir was increased from 25 ºC to 85 ºC gradually to study the effects of the temperature, which is depicted inFigure 6 (b). The results show that the Vocgradually increases up to the voltage of 720 mV after a certain time while the temperature reaches 85 ºC. This can be attributed to the combined effects of the enhanced movement of water molecules, elevated evaporation rate, and thermoelectric effects.[50]In addition, five different heights of the shell structure were examined to see the thickness effects on the Voc. Since the thickness of the WS was under 2 mm, the cross-sectional configurations were considered for the elevated heights experiments. The density along the cross-section is comparatively higher, confirmed by SEM (Figure-S 3); therefore, the slightly lower voltage was observed. The results inFigure 6 (c) confirm that altered heights have a substantial effect on the Voc. Elevated heights enhance the variations of ion concentrations; however, the more restrictions on water flow along the elevated cross sections reduce the output voltage.
By swapping the positive and negative electrodes on the multimeter, the polarity and magnitude of the open circuit voltage were compared. The results of Figure 6 (d) reveal that changing the polarity has no major impact on the voltage; the polarity changes only caused by the flipping. The effect of the humidity on the device performance was evaluated systematically. The results shown in Figure 6 (e) indicate the lower performance in extremely humid environments. This is due to the reduced evaporation rate in humid environments. While evaluating the effects of different concentrations of NaCl, the open circuit voltage seems to be higher at higher concentrations of NaCl, which is represented in Figure 6 (f) . Compared to water, NaCl leads to an enhanced voltage and current density of 703 mV and 0.78 µW/mm2 respectively. Theoretically, enhanced concentrations of ions lead to a decrease in streaming potentials because of the reduced size of the double layer.[19] In this case, however, enhanced open circuit voltage are caused by the continuous evaporations of water. The higher solubility of NaCl and elevated migration rate of Na+ ions associated with WS micro/nanochannels facilitate the higher concentration gradients in the ionic pathway.[51] The output voltage is, consequently, therefore, boosted by the enhanced level of ions in the solutions.

Performance enhancement and power density analyses

The acid-treated samples (WS-H+) were immersed in the reservoir of neutral water and alkaline solutions separately as illustrated in Figure 7 (a). WS-H+experiences a partial reduction of lignin and exposure of cellulose as observed in CLSM (Figure 3 (f)) and FT-IR spectra (Figure 7 (b) ). As a result of this treatment additional nanochannels are created, as confirmed by the SEM (Figure 3 (h) ), which can be referred to as nano-engineered WS-H+. C-O-C asymmetric stretching at 1160 cm-1, C-H bending at 1360 cm-1 and C-O-C stretching at 1105 cm-1 confirms the presence of cellulose and C=O at 1040 cm-1 confirms the stretching vibration of cellulose and hemicellulose[52] which are prominent in the nano-engineered WS-H+ samples as confirmed by Figure 7 (b).
Upon acid treatments the WS-H+ samples were protonated and the signals at 1738 cm-1 disappeared after being placed on alkaline reservoir for 8 h as shown in Figure-S 9 . The thermal gravimetric analyses at Figure 7 (c) demonstrates that the thermal stability of the natural walnut shell (NWS) is more than the treated WS and WS-H+. This is due to the reduction of lignin contents after the chemical treatments since lignin has higher thermal stability because of its complex heterogeneous structure.[53]